That's because RAW is a file format
not
an image format. A RAW file only provides a histogram once conversion settings are applied to the RAW data. If you want to see what's in your RAW file without lots of contrast settings added, choose as conservative a image/picture style as possible that has the least contrast, saturation, and least steep tone curve applied.

I can't wholly agree conceptually. NEF contains image data, which clearly is "histogramizible", check Photobola Rawnalyze. But it is true that it makes sense to apply a "picture control" (tone curve) to make the histogram more "legible". UniWB also produces an approximation of the raw histogram for the highlights end, but it certainly is clumsy. It is a big question how this should be implemented in a camera to make it most useful and not disturbing for certain groups of users. One option I've seen somewhere was to extend the RGB histogram on both ends, so that any spikes indicating blown highlights (right) or shadows (left) are expanded to indicate the content of the raw headroom in white and the size of the headroom for each channel.

Notice mostly no PC is conservative enough - Nikon's processing is not designed for processing high DR. I guess considerable modifications are required to change this.